Post-synthetic modification of semiconductor nanoparticles can generate lanthanide luminophores and modulate the electronic properties of preformed nanoparticles

Saoni Rudra; Madhumita Bhar; Prasun Mukherjee

doi:10.1051/fopen/2023007

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Volume 6 (2023)

4open, 6 (2023) 8

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Inorganic Nanoparticle Luminophore: Design and Application

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Review

Issue		4open Volume 6, 2023 Inorganic Nanoparticle Luminophore: Design and Application


Article Number		8
Number of page(s)		20
Section		Chemistry - Applied Chemistry
DOI		https://doi.org/10.1051/fopen/2023007
Published online		03 August 2023

4open 2023, 6, 8

Review Article

Post-synthetic modification of semiconductor nanoparticles can generate lanthanide luminophores and modulate the electronic properties of preformed nanoparticles

Saoni Rudra, Madhumita Bhar and Prasun Mukherjee^*

Centre for Research in Nanoscience and Nanotechnology, University of Calcutta, JD-2, Sector-III, Salt Lake, Kolkata 700106, West Bengal, India

*Corresponding author: pmcrnn@caluniv.ac.in, pmukherjee12@gmail.com

Received: 30 June 2022
Accepted: 27 June 2023

Abstract

Post-synthetic modification of inorganic nanoparticles (NPs) provides a unique lesser synthetically demanding opportunity to access nanomaterials those are oftentimes not directly realizable by conventional synthetic routes. Trivalent lanthanide (Ln³⁺) incorporated (doped) semiconductor NPs can benefit from individual properties of the NPs and Ln³⁺ moieties. This work summarizes key outcomes from experiments when (a) ZnS /CdS /CdSe NPs are post-synthetically treated with Ln³⁺ to generate ZnS/Ln or CdSe/Ln [Ln = Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb] and CdS/Ln [Eu, Tb] NPs, (b) synthetically Tb³⁺ doped Zn(Tb)S NPs are post-synthetically modified with varying concentration of heavy metals like Pb²⁺/Cd²⁺ to generate Zn(Tb)S/M [M = Pb, Cd] NPs, and (c) the pH of Zn(Tb)S NPs aqueous dispersion is varied post-synthetically. Key observations from these experiments include (a) incorporation of Ln in all the post-synthetically prepared CA/Ln NPs, with presence of host sensitized dopant emission in select cases that can be rationalized by a charge trapping mediated dopant emission sensitization processes, (b) existence of rich photophysics in the sub-stoichiometric reactant concentration ratio, and (c) identifying the alteration of surface capping ligand structure as an important variable to control the Ln³⁺ emission. In summary, these experimental observations provide an easy control of reaction conditions either to generate Ln³⁺ inorganic NP luminophores or to control their electronic properties by modulating either the NP’s core or surface properties, and are of potential usefulness in various luminescence based applications.

Key words: Semiconductor nanoparticles / Lanthanides / Post-synthetic modification / Doping / Luminescence

© S. Rudra et al., Published by EDP Sciences, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Introduction

Inorganic nanoparticles (NPs) offer a wide range of potential in both fundamental and application perspective. Synthesizing them by conventional bottom-up approaches like hot injection, [1, 2] heating up, [3, 4] sol–gel solvothermal [5, 6] methods are known. These synthetic protocols are established to generate materials in quantum confined regime, thus availing various unique properties. Besides these conventional synthetic routes which are time consuming, a unique way to tune the properties of nanomaterials is to treat them post-synthetically. That is, modifying an inorganic NP by treating them following their synthesis. An extremely fascinating avenue to this front is demonstrated by Alivisatos and co-workers [7] in 2004, where these researchers discussed feasibility of cation exchange with the formation of Ag₂Se NPs by treating CdSe NPs with Ag⁺ at room temperature. In this reaction the incoming Ag⁺ and outgoing Cd²⁺ exchange their spatial positions without perturbing the anionic framework of the nanocrystal. Furthermore, by adding excess of Cd²⁺ and adjusting the chemical nature of the surface capping ligand a reverse cation exchange is found to be possible. This outstanding observation opened up a ground to access a wide range of nanomaterials those are oftentimes not directly realizable using conventional synthetic protocols. Additionally, these reactions are facile and generally performed at or near ambient conditions, reduce much synthetic efforts and time, and are cost effective, as various nanomaterials can be accessed from a single batch of initial reactant NP synthesis. These reactions furthermore provide an easy comparison of physico-chemical properties of the reactant and product NPs. Comparison of NP’s properties from different synthesis face challenges due to inherent batch to batch inhomogeneity.

This outstanding observation by Alivisatos and co-workers triggered related efforts in various research laboratories. Some of the demonstrated cation exchange by post-synthetic modification reactions include formation of CdS–Ag₂S nanorods [8], CdS–Cu₂S nanorods with a different morphology than that of CdS–Ag₂S nanorods [9], CdSe/CdS → Cu₂Se/Cu₂S → PbSe/PbS nanorods, [10] III–V nanocrystals (InP, InAs, GaP, and GaAs) [11], CdSe (core) – CdS (pods) → CdSe (core) – CdS/Cu₂S (pods) → Cu_2-xSe (core) – Cu₂S (pods) [12], CdSe → Cu₂Se → ZnSe with size, shape and crystal structure conservation of the nanocrystals [13], alloyed quaternary and quinary copper chalcogenide nanocrystals [14], Cu_2-xSe → Cu₂SnSe₃ and SnSe by addition of Sn⁴⁺ and Sn²⁺ respectively [15], Cu₂Te → CdTe [16], partially and completely exchanged CdSe/Ag and Ag₂Se nanoplatelets [17], InAs → InAs/Cu, InAs/Ag, InAs/Au [18], CdSe → CdSe/Ag [19], and LnF₃ → Ln′F₃ with atomic number of Ln > Ln′ [20]. These studies clearly point on the robustness of using cation exchange reactions to access nanomaterials with suitable incoming and outgoing cation pairs of differing size and charge. Few review articles are currently available discussing various facets of cation exchange reactions, covering demonstrated reactions with mechanistic insights based on thermodynamic and kinetic considerations [21–26].

Another scenario may arise where the incoming – outgoing cation pairs do not take part in exchange, but somehow can still alter the properties of the nanomaterial by altering the NP’s surface properties. Such examples are discussed in the examination of spectra of Zn(Mn)S and ZnS/Mn NPs by Murphy and co-workers [27], where these researchers reported a dramatic difference in photophysical outcome between these two assemblies. While the synthetically doped Zn(Mn)S NPs give rise to blue ZnS centered and orange Mn²⁺ centered emissions, the ZnS/Mn NPs yield a blue shifted ZnS emission without Mn²⁺ signal. Similarly, Chrysochoos and co-workers [28] discussed modulation of CdS NP’s surface properties by addition of Cu²⁺ post-synthetically. Collectively, post-synthetic modification of inorganic NPs with a suitable cation pair can result in cation exchange, and even when this is absent mere alteration of NP’s surface state composition may also be able to access modified properties.

Doping an inorganic NP has the ability to tune the chemical, optical, magnetic, electrical, electronic properties; those are otherwise not directly accessible in the undoped NP [29, 30]. Of particular interest are the d- and f-block elements to generate optical materials with interesting emission properties. Presence of suitable dopants among these elements can give rise to additional emission bands compared to that from the NP excitonic and/or surface state emissions. Emissions from d-block elements are generally broad due to efficient mixing with ligand orbitals relaxing the spin selection rule. On the other hand, emissions from f-block elements are unique. These bands are sharp with minimum intra- and inter-Ln³⁺ spectral overlap, span the entire visible and near infrared (NIR) spectral range, longer (microseconds–milliseconds) lifetime allowing time-gated measurements, relatively insensitive to environment, resistant to photobleaching thus allowing longer experiment time and enhancing the signal-to-noise ratio. Lanthanide containing luminophores thus find unique applications in biological imaging, bio-analytical applications, optoelectronics, sensing, lasers, and telecommunications; among others [31–45].

Accessing useful emissions from Ln³⁺ is however challenging due to their inefficient direct excitation [ε (Ln³⁺) ≤ 10 M⁻¹ cm⁻¹, ε (organic fluorophore) ≈ 10⁴–10⁵ M⁻¹cm⁻¹] and environmental vibrational overtone induced emission quenching [34, 46]. One way to address these challenges is to incorporate Ln³⁺ in an appropriate coordination environment, in which by virtue of an efficient excitation of the surrounding entities, energy can be transferred to the Ln³⁺ center. That is, an optical antenna effect can operate to sensitize the Ln³⁺ emission. Various research laboratories are devoting attention to synthesize Ln³⁺ containing inorganic molecules. [47–50] On the other hand, similar beneficial scenario can also be visualized by incorporating (doping) an appropriate Ln³⁺ in a suitable semiconductor NP, in which the NP can act as an optical antenna to sensitize the Ln³⁺ luminescence. These inorganic NPs also act as a protector matrix by offering lower frequency vibrations, and thus environment induced non-radiative processes are largely suppressed [51]. Additionally, these composite host (semiconductor NP) – guest (Ln³⁺) assemblies benefit from individual properties of the ingredients, and provides avenues of accessing quantum confinement, surface engineering, inclusion of multiple and distinct Ln³⁺ in a given NP. These properties can be explored for targeted therapy and multiplex assays.

During early developments in this field, Bol and co-workers raised an important question on the feasibility of incorporating Ln³⁺ in semiconductor NPs considering the size and charge mismatch of the ingredient cations [52]. Later studies by various research groups demonstrated that an optimization of synthetic protocol can indeed incorporate Ln³⁺ in inorganic NPs [53–60]. In an early seminal demonstration in this research direction, Petoud and co-workers discussed successful Tb³⁺ doping in CdSe NPs [53], in which the NPs are demonstrated to act as the feeding channel to sensitize the Tb³⁺ emission and offer spatial protection from environmental non-radiative deactivation processes. Other such demonstrations include incorporation of Eu³⁺ in TiO₂[5], Nd³⁺/Sm³⁺ in TiO₂, [6] Er³⁺ in SnO₂ [61], Eu³⁺ in SnO₂ [62], Er³⁺ in TiO₂ [63], Tb³⁺ in ZnS, [64, 65] Eu³⁺ in ZnS [66–74], Eu³⁺ in CdS [75], Eu³⁺/Tb³⁺ in CdSe–ZnS [76], Yb³⁺ in surface modified CdSe [77]. The size and charge mismatch can be compensated by lattice distortion and charge compensation. Based on site symmetry consideration, Chen and co-workers discussed how incorporation of Eu³⁺ in TiO₂ NPs can change the Ti⁴⁺ symmetry of core sites from D_2d to either D₂ or C_2v and surface sites to C₁ symmetries [5]. Aliovalent doping of Cu²⁺/Ag⁺ in InAs NPs is also characterized by Banin and co-workers [18]. Motivated by the unconventional emission characteristics of Ln³⁺, we initiated working on developing Ln³⁺ doped semiconductor NPs and understanding the underlying photophysical processes, in order to access materials with predictable and desirable properties. In 2011, Waldeck, Petoud and co-workers reported an analysis to rationalize the photophysical outcome of Tb³⁺ and Eu³⁺ emissions in II-VI sulfide and selenide materials [78]. The experimental observations were rationalized based on a charge trapping mediated sensitization model, in which appropriately positioned Ln³⁺ ground and luminescent energy levels can act as potential hole and electron traps, and the electron-hole pair recombination at these trap sites populates the Ln³⁺ luminescent energy levels thus generating host sensitized dopant emission. This analysis also identifies ZnS being the most efficient host matrix in order to achieve maximum host sensitization. It is further demonstrated that a smaller dimension of NP is beneficial to maximize the sensitized Tb³⁺ emission in ZnS based NPs [79]. We also argued that the spectral overlap of donor NP emission and acceptor Ln³⁺ absorption is insignificant in deciding the spectral outcome of these assemblies [78–81].

These outcomes on synthetically doped Ln³⁺ semiconductor NPs combined with the robustness of post-synthetic modification of inorganic NPs led us to undertake investigations with Ln³⁺ being the incoming cation. These studies are categorized as CA/Ln systems, where Ln after slash indicates their addition post-synthetically to pre-formed NPs. Another obvious scenario arises from treating synthetically Ln doped, C(Ln)A that is Ln mentioned within parenthesis, NPs with Mⁿ⁺ to synthesize C(Ln)A/M NPs, and to study how the inclusion of M can tune the photophysical properties of the NP. Different outcomes that can emerge from this reaction formulation include modulation of NP’s electronic properties without or with alteration of the NP’s core structure. Finally, presence of a surface capping ligand on NPs confers stability and guide NP’s dispersion nature. In the photophysical perspective, these capping ligands are known to guide NP’s emission extent [82], reaction kinetics [83–85], dopant emission lifetime [86]; and often contains ionisable groups. Such a NP, in the form of 1-thioglycerol (1-TG) capped Zn(Tb)S, is treated post-synthetically to vary the dispersion pH, and its photophysical properties was investigated. Collectively, these experiments yield a comprehensive picture to generate Ln³⁺ semiconductor NP, tune properties of existing Ln³⁺ by either modulating the core or surface of the NPs by using the facile post-synthetic modification strategy. The plan of this perspective with these different facets is summarized in Scheme 1. Population of Ln³⁺ in both core and surface sites of the NPs is evident from the experimental observations (see Discussion below). Both substitutional and interstitial doping is possible for core sites. These aspects are not shown explicitly while constructing the Scheme 1. Future perspectives are also discussed.

Scheme 1

Different post-synthetic reaction strategies (RT stands for room temperature).

Results and discussion

I. CA/Ln NPs: generation of Ln³⁺ doped semiconductor NP luminophores

(i) ZnS/Ln [Ln = Tb, Eu]. The knowledge of ZnS NPs’ ability to sensitize synthetically doped Tb³⁺ and Eu³⁺[78] led a platform to treat ZnS NPs post-synthetically with Tb³⁺/Eu³⁺. In 2013, Waldeck, Petoud and co-workers reported the observations from experiments in which the ZnS NPs were treated post-synthetically by adding Tb³⁺/Eu³⁺ nitrates, with their quantities matching the corresponding synthetically doped cases [87]. Most interestingly, noticeable Tb³⁺ and Eu³⁺ emissions are observed from the ZnS/Tb and ZnS/Eu NPs. The emission spectra of the ZnS, ZnS/Tb, and ZnS/Eu NPs are shown in panel (b) of Figure 1. Clearly, in the ZnS/Tb NPs sharp Tb³⁺ emission bands are visible that are centered at 490, 545, 585, and 620 nm that originates from the ⁵D₄→⁷F_n [n = 6–3] transitions, in addition to the broad blue emission of ZnS emissive center. Similarly, the sharp Eu³⁺ emissions appear at 590, 616, and 700 nm that originates from the ⁵D₀→⁷F_n [n = 1, 2, 4] transitions. The excitation spectra, while monitoring the Ln³⁺ emission bands are shown in panel (a) of Figure 1. Corresponding spectra for the undoped ZnS NPs are also included. These spectra primarily reveal a broad profile that overlap significantly with that of the NPs, without significant contributions from direct sharp 4f–4f bands. This unequivocally suggests that the ZnS NPs act as a feeding channel to sensitize either Tb³⁺ or Eu³⁺ emissions. That is, an optical antenna effect operates in these assemblies. The post-synthetically modified ZnS/Tb NPs (taken as a representative system) are stable over time. Acquisition of emission spectra even after ~2.5 months of post-synthetic modification shows no degradation of the Tb³⁺ emission. Panel (c) of Figure 1 shows the trend in Eu³⁺ emission when gradually ZnS NPs are added to the solution of Eu³⁺ nitrates in chloroform. An increase in Eu³⁺ emission is observed that is correlating with the amount of NPs present in the chloroform dispersion.

Figure 1

Luminescence excitation and emission spectra of the ZnS, ZnS/Tb, and ZnS/Eu NPs are shown in panels (a) and (b), respectively. Panel (c) shows the Eu³⁺ emission trend when ZnS NPs are gradually added to the dispersion. The Ln³⁺ emission lifetime decay profiles in the ZnS/Tb and ZnS/Eu NPs are shown in panel (d), with the corresponding profiles of freely floating Ln³⁺ included. Luminescence excitation and emission spectra of the post-synthetically treated CdS/Tb and CdS/Eu NPs are shown in panels (e) and (f), respectively. The electronic absorption spectrum of the CdS NPs is also included in panel (e) for comparison. Panels (g), (h), and (i) show the trends in Eu³⁺ asymmetry ratio, emission lifetime, and emission lifetime distribution, respectively. (Adapted with permission from Reference [87], Copyright 2013 American Chemical Society).

In order to evaluate the extent of spatial protection that these NPs can offer to the Ln³⁺, the Ln³⁺ emission lifetimes in the ZnS/Tb and ZnS/Eu NPs are acquired, and the corresponding decay profiles are compared to that obtained for the freely floating Tb³⁺/Eu³⁺ in bulk solvent, which are shown in panel (d) of Figure 1. The lifetimes for Tb³⁺ and Eu³⁺ emissions in bulk solvent are found to be 250 and 125 μs, respectively. In presence of ZnS NPs, these lifetimes are significantly lengthened, and they are found to be bi-exponential in nature. Specifically, Tb³⁺ and Eu³⁺ emission lifetime components are found to be 500 μs and 2.4 ms, and 500 μs and 1.5 ms, for the ZnS/Tb and ZnS/Eu NPs, respectively. These two lifetime components can arise from different spatial location of Ln³⁺ in the NPs. That is, while the surface localized Ln³⁺ are more prone to environment induced non-radiative deactivation pathways generate a shorter lived emission, the core localized Ln³⁺ can produce much longer lifetime due to better spatial protection. In well protected molecular complexes, Tb³⁺ and Eu³⁺ emission lifetimes were found to be 1.3 and 0.78 ms, respectively [88]. This clearly indicates that the Tb³⁺/Eu³⁺ emissions are well protected in the ZnS NPs even when the Ln³⁺ precursors are added post-synthetically to the pre-formed NPs.

(ii) CdS/Ln [Ln = Tb, Eu]. Generality of these observations and usefulness of this post-synthetic modification strategy to generate Ln³⁺ doped inorganic NP luminophores is examined by undertaking experiments in which the CdS NPs are post-synthetically treated with Tb³⁺/Eu³⁺ separately. The emission spectra from these measurements are shown in panel (f) of Figure 1. Remarkable enhancement of Tb³⁺/Eu³⁺ emissions are observed in presence of the NPs, compared to that observed in the freely floating Tb³⁺/Eu³⁺. The excitation spectra, while monitoring the Tb³⁺/Eu³⁺ emissions are shown in panel (e) of Figure 1. Corresponding electronic absorption spectrum is also included. Similar to the ZnS NPs case, these profiles are also broad without noticeable contributions from direct sharp 4f–4f excitation bands. Most remarkably, these spectra identify bands at the lowest energy excitonic band maxima of the CdS NPs centered at 360 nm. Thus, these spectra affirmatively confirm operation of an optical antenna effect to sensitize Tb³⁺/Eu³⁺ emissions by the CdS NPs.

(iii) Role of Spatial Position: A Case Study with Eu³⁺ Doped NPs. It is important to evaluate the role of Ln³⁺ spatial position in deciding the photophysical outcome in the CA/Ln NPs. Correspondingly a comparative analysis is undertaken between the ZnS/Eu and Zn(Eu)S NPs: in the form of asymmetry ratio (ratio of integrated emission intensity of the Eu³⁺ electric and magnetic dipole transitions centered at 616 and 590 nm, respectively) [34, 89], emission lifetime, and lifetime distribution analysis. A summary of these analyses are presented in the panels (g)–(i) of Figure 1. The intensity of electric dipole transition increases in an asymmetric environment, while the intensity of the magnetic dipole transition remains unaffected. Correspondingly, an increase in asymmetry ratio can be correlated with an asymmetric co-ordination environment around Eu³⁺. The Eu³⁺ asymmetry ratio values are found to be 4.8 ± 0.1, 5.5 ± 0.6, and 5.6 ± 0.1 in the Zn(Eu)S, ZnS/Eu, and Zn(Eu)S/Eu NPs, respectively. Thus, a more asymmetric environment is experienced by Eu³⁺ when they are added post-synthetically, compared to the synthetically doped NPs. In corroboration to the trends in asymmetry ratio values, the contribution of shorter surface related lifetime component increases in the post-synthetically added NPs, compared to that in the synthetically doped Zn(Eu)S NPs. A more realistic scenario is to visualize Ln³⁺ being randomly distributed in the NPs, rather discretely localized at specific core and surface sites of the NPs. Lifetime distributions are hence compared. Clearly, contributions of shorter lifetime components are increased in the Zn(Eu)S/Eu compared to that in the Zn(Eu)S NPs. These data cumulatively indicate a small yet discernible enhanced surface population of Eu³⁺ in the post-synthetically treated NP, compared to that in the synthetically doped counterparts.

(iv) CdSe/Ln and ZnS/Ln [Ln = Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb]. While the experiments described above clearly demonstrate the usefulness of post-synthetic reaction strategy to develop Ln³⁺ containing semiconductor NPs, these data do not identify the (a) extent of Ln incorporation in the NPs and more specifically the fate of NP’s elemental composition due to the introduction of Ln, (b) feasibility of realizing host sensitized emissions in other Ln³⁺ besides Tb³⁺ and Eu³⁺, and (c) hence unravelling the underlying photophysical processes. In order to address these questions more comprehensively, we performed experiments in which the CdSe and ZnS NPs were treated post-synthetically by adding an excess (100 and 5 times for the CdSe and ZnS NPs, respectively) amount of Ln³⁺ [Ln = Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb] nitrates [90]. Post-synthetic modification of the CdSe NPs with diameter of 2.2 ± 0.3 nm by Tb³⁺ resulted in the CdSe/Tb NPs with a diameter of 2.6 ± 0.4 nm. Similarly, corresponding values for the ZnS and ZnS/Tb NPs were found to be 2.2 ± 0.4 and 2.5 ± 0.4 nm, respectively. We note that the NPs dimension are almost very similar following the post-synthetic addition of Ln³⁺ to either the CdSe or ZnS NPs, and the change in NP’s diameter of ~15% does not have significant impact on their colloidal stability.

(a) Elemental Composition. The elemental composition of the CdSe/Ln and ZnS/Ln NPs, as obtained from the energy dispersive X-ray spectroscopy (EDS), are shown in panels (a) and (b) of Figure 2, respectively. All the CdSe/Ln and ZnS/Ln NPs capture incorporation of Ln in the NPs. However, a difference in extent of Ln introduction is noticed as a function of host. While CdSe incorporates them in smaller amounts, their values are higher in the ZnS NPs. Complete cation exchange of either Cd²⁺/Zn²⁺ by Ln³⁺ is not evident. Moreover, significant alteration in the anionic framework is noticed upon addition of Ln³⁺ to these NPs.

Figure 2

Elemental composition of the CdSe/Ln and ZnS/Ln, as determined from the EDS measurements are shown in panels (a) and (b), respectively. (Adapted with permission from Reference [90], Copyright 2019 Elsevier).

A simple estimation of predicting the feasibility of Cd²⁺/Zn²⁺–Ln³⁺ exchange on thermodynamic considerations [21] gives rise to a reaction free energy value that is close to zero [90], suggesting that this cation exchange is not a favorable process. Thus, observation of absence of complete exchange of Cd²⁺/Zn²⁺ by Ln³⁺ is justifiable. A remarkable alteration of the anionic framework of the NPs in both the cases are noteworthy, with formation of cation rich NPs. The diffusion co-efficient of Cd²⁺, Zn²⁺, and Ln³⁺ are 0.72 × 10⁻⁵ cm² s⁻¹, 0.70 × 10⁻⁵ cm² s⁻¹, and (0.58–0.62) × 10⁻⁵ cm² s⁻¹, respectively. This suggests that the inward cation diffusion of Ln³⁺ is slower compared to that of the outward diffusion of Cd²⁺/Zn²⁺ from the NPs. That is, a nanoscale Kirkendall effect operates [23, 91, 92]. The diffusion co-efficient of S^2- is 0.73 × 10⁻⁵ cm² s⁻¹. In this post-synthetic modification process of the NPs, Zn²⁺ extracts S^2-. This most likely associates with greater affinity of Zn²⁺ to S²⁻, compared to that of Ln³⁺. This reorganization of NPs is probably kinetically favored.

(b) Luminescence. The steady-state emission spectra of the CdSe and CdSe/Ln [Ln = Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb] NPs are shown in panels (c) and (d) of Figure 3. CdSe NPs show an excitonic band maximum at 510 nm. Addition of Ln³⁺ diminishes this band to a significant extent, and only a small contribution is noticed. This is consistent with partial cation exchange of Cd²⁺ by Ln³⁺ in generating the CdSe/Ln NPs. Introduction of Ln³⁺ in the NPs generally alters the emission characteristics, with a blue shifted much broader emission contribution. Among all the CdSe/Ln NPs investigated, only Tb³⁺ and Eu³⁺ show sensitized emission. This can be tracked by monitoring the excitation spectra, as shown in panel (a) of Figure 3. While monitoring the respective emission bands these spectra yield a broad spectrum without contributions from sharp direct 4f–4f bands, suggesting operation of an optical antenna effect. Furthermore, similar to the CdS/Ln [Ln = Tb, Eu] NPs discussed above, these excitation spectra capture signature of first excitonic absorption band, strengthening the argument of NPs induced sensitization effect.

Figure 3

The luminescence excitation and emission spectra of the CdSe/Ln and ZnS/Ln [Ln = Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb] NPs are shown in panels (a)–(g), respectively. The electronic absorption spectra of the CdSe and ZnS NPs are also included in panels (a) and (b) for comparison. The shorter time decay kinetics of the ZnS and ZnS/Ln [Ln = Tb, Eu, Pr, Ho] to monitor the NP’s intrinsic depopulation rates are shown in panel (h). Panel (i) shows the longer time decay profiles of the CdSe/Ln and ZnS/Ln [Ln = Tb, Eu] NPs to monitor the Tb³⁺ and Eu³⁺ depopulation rates. Panel (j) shows the lifetime distribution analysis of the Tb³⁺ and Eu³⁺ emissions. (Adapted with permission from Reference [90], Copyright 2019 Elsevier).

The corresponding emission spectra in the ZnS/Ln [Ln = Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb] NPs are shown in panels (e) and (f) of Figure 3. ZnS NP’s emit at blue spectral range with a maximum around 420 nm. This broad blue emission is correlated primarily with sulfur vacancies, however contributions from zinc and sulfur interstitials and zinc vacancies is also discussed. [27, 93–95] Higher and lower energy contributions originate from the roles of interstitial and vacancy sites, respectively. Introduction of Ln in the ZnS/Ln NPs do not alter this surface state composition to a significant extent. Among the ZnS/Ln NPs, Tb³⁺, Eu³⁺, and Yb³⁺ show sensitized emission. The Yb³⁺ emission from the ZnS/Yb NPs is shown in panel (g) of Figure 3. The sensitization effect is confirmed by acquiring the respective excitation spectra while monitoring the Ln³⁺ emission bands. These spectra are shown in panel (b) of Figure 3. While generating the CdSe/Ln and ZnS/Ln NPs, the CdSe or ZnS NPs were post-synthetically treated with an excess amount of incoming Ln³⁺ cations, to drive the reaction as much as possible to the product side. Thus these reactions do not directly probe the doping extent dependence on the optical properties of these NPs. A systematic investigation to decipher this aspect would be extremely interesting and we consider this as a future direction.

The shorter time dynamics of the ZnS and ZnS/Ln [Ln = Tb, Eu, Pr, Ho] NPs are measured by exciting the NPs at 290 nm and collecting the broad ZnS centered emissions at 400 nm. These decay profiles are shown in panel (h) of Figure 3. All these decay profiles are similar, with an average emission lifetime in the range of tenths of picoseconds. This suggest that operation of sensitization in Tb³⁺/Eu³⁺ cases do not impact ZnS decay kinetics. Collecting the emission at 545 nm, a wavelength where Tb³⁺ emits, only shifts the baseline and produces lifetimes in the range of picoseconds. The Ln³⁺ emission lifetimes in the CdSe/Ln and ZnS/Ln [Ln = Tb, Eu] are measured with a set-up having lower repetition rate of the excitation source to monitor the longer time decay dynamics. These decay profiles are shown in panel (i) of Figure 3. All these decay profiles reveal bi-exponential decay kinetics, where the shorter and longer lifetime decay components are correlated with the lesser and more protected surface and core sites of the NPs, respectively. The lifetime distribution analysis from these decays is shown in panel (j) of Figure 3. The lifetime decays and their distribution analyses suggest that CdSe NPs offer better spatial protection to the Tb³⁺ / Eu³⁺ emissions. The radiative rates of NP are approximately million times faster than that of the Ln³⁺ emissive centers, thus they remain unaffected even in presence of Ln³⁺ sensitized emissions from the doped NPs.

(c) Photophysical Rationalization. In the context of realizing host sensitized Ln³⁺ emissions, it is remarkable to note that this is only observed in the CdSe/Tb, CdSe/Eu, ZnS/Tb, ZnS/Eu, and ZnS/Yb NPs. Other NPs among the CdSe/Ln and ZnS/Ln [Ln = Pr, Nd, Sm, Dy, Ho, Er, Tm] do not show host sensitized Ln³⁺ emissions. These observations are rationalized within a charge trapping mediated dopant emission sensitization mechanism. The Ln³⁺ energy levels are placed with respect to the NP’s valence and conduction bands following a method proposed by Dorenbos and co-workers [96, 97] and later adopted by us. [78–81, 90, 98]

The key assumptions in this semi-empirical approach are as follows. (i) The binding energy trend of Ln³⁺ is universal and is host independent. This is by virtue of the core like feature of the 4f electrons. (ii) The charge transfer energy from the anion valence band to Eu³⁺ is equal to the energy difference between the valence band of the host material and the Eu²⁺ ground energy level in that material, and (c) the energy difference between the Eu²⁺ and Eu³⁺ ground energy levels in lower band gap materials is equal to 5.7 eV. With these inputs and known band gap value of a host material, all the Ln³⁺ and Ln²⁺ ground energy levels can be placed with respect to the valence and conduction bands of the host material. The higher lying Ln³⁺ energy levels can be placed accordingly following the reports by Rajnak and co-workers [99–101]. Such energy level schematics are constructed for the CdSe/Ln and ZnS/Ln [Ln = Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb] NPs, and are shown in Figure 4.

Figure 4

Relative energy level schematic with placement of Ln³⁺ ground and luminescent energy levels with respect to the valence and conduction bands of the host material is shown. Ground and luminescent energy levels in each case are represented by black and blue solid lines, respectively. Prominent luminescence transitions are shown with downward arrows in each case. Ln²⁺ ground energy levels are indicated by green horizontal lines. These energy levels are positioned following a method proposed by Dorenbos [96, 97] and later adopted by us, [78–81, 90, 98] and relies on inputs of host independent universal binding energy trend of Ln³⁺ and the charge transfer energy from anion valence band of host material to Ln³⁺ moieties. (Adapted with permission from Reference [90], Copyright 2019 Elsevier).

Important energy offsets in this model are the energy differences between the top of the valence band and the Ln³⁺ ground energy level (ΔE ₁) and between the bottom of the conduction band and the Ln³⁺ luminescent energy level (ΔE ₂). An optimum negative and positive value of ΔE ₁ and ΔE ₂ ensure effective co-localization of photogenerated hole and electron at the Ln³⁺ ground and luminescent energy levels, respectively. Subsequent electron-hole pair recombination at the Ln³⁺ related trap site populates the Ln³⁺ luminescent energy level, thus producing host sensitized dopant emission from the doped NPs. It is important for these energy offsets to be optimum, as a lower and higher value would result in back charge transfer and experience detrimental effects from competitive non-radiative relaxation processes. Another scenario may arise when these energy levels are buried in one of the bands. For example, if the luminescent energy level of a particular Ln³⁺ lies above the conduction band of the host material, then the photogenerated electron would experience auto-ionization effect, thus reducing the Ln³⁺ emission efficiency. These different scenarios are summarized in Scheme 2.

A close inspection of the energy level schematic in Figure 4 reveals a favourable energy level alignment in Tb³⁺, where the ground and luminescent energy levels ⁷F₆ and ⁵D₄ are positioned suitably above and below the valence and conduction bands of both CdSe and ZnS NP host. In cases of Nd³⁺, Sm³⁺, Eu³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺ the ground energy level is buried in the valence band, thus they are prone to experiencing auto-ionization of the photogenerated hole. In case of Pr³⁺ ΔE ₁ is marginally negative, thus also contributing detrimentally. Thus, only Tb³⁺ ground energy level can act as a potential hole trap in either CdSe/ZnS NP host. The values of ΔE ₂ is very large in cases of Nd³⁺, Sm³⁺, Eu³⁺, Dy³⁺, Ho³⁺, Er³⁺, Yb³⁺, implying roles of competitive relaxation pathways through NP’s various surface states. This ΔE ₂ value is not severely adverse in cases of Pr³⁺ and Tm³⁺. An additional energy level of importance comes in picture for Sm³⁺, Eu³⁺, Tm³⁺, Yb³⁺ where the respective Ln²⁺ ground energy levels lie within the band gap. The relevant energy difference, ΔE ₃, is between the conduction band of the host NP and the Ln²⁺ ground energy level. An optimum positive value of ΔE ₃ offers an additional excitation pathway to populate the Ln³⁺ luminescent energy levels by the involvement of Ln²⁺ ground energy levels. The relevance of ΔE ₁, ΔE ₂, and ΔE ₃ in CdSe and ZnS NPs are summarized in Table 1.

Table 1

Summary of relevance of ΔE₁, ΔE₂, and ΔE₃ in CdSe and ZnS NPs.

Besides the efficiency of trapping photogenerated charge carriers at Ln³⁺ related energy levels, another important consideration comes from the environment induced emission quenching of Ln³⁺. In this regard, the energy difference between the luminescent energy level and the highest spin orbit level of the ground energy level is of importance. This energy difference is represented as ΔE ₄, and the ΔE ₄ values for different Ln are tabulated in Table 2. A lower value of ΔE₄ correlates to higher quenching efficiency.

Table 2

Energy difference considerations for different Ln related to environment induced emission quenching.

Based on the photophysical outcome, the following steps are proposed for the Ln³⁺ emission sensitization in semiconductor NPs, where the Ln³⁺ ground and luminescent energy levels are optimally placed above and below the valence and conduction band of the host NPs.

(1)
Excitation of semiconductor NP and generation of electron-hole pair

$Z n (L n) S + h ν \to Z n S [h^{+} + e^{-}]$ $\mathrm{Z}\mathrm{n}(\mathrm{L}\mathrm{n})\mathrm{S}\mathrm{ }+\mathrm{ }\mathrm{h}\mathrm{\nu }\mathrm{ }\to \mathbf{ }\mathrm{Z}\mathrm{n}\mathrm{S}\mathrm{ }[{\mathrm{h}}^{+}\mathrm{ }+\mathrm{ }{\mathrm{e}}^{-}]$
(2)
Hole trapping at the dopant site followed by electron trapping from a non-dopant site and creation of the excited Ln³⁺

$\begin{matrix} {L n}^{3 +} + h^{+} \to [{L n}^{3 +} + h^{+} ⟺ {L n}^{4 +}] \\ [{L n}^{3 +} + h^{+} ⟺ {L n}^{4 +}] + e^{-} {\to L n}^{3 + *} \end{matrix}$ $\begin{array}{c}{\mathrm{L}\mathrm{n}}^{3+}\mathrm{ }+\mathrm{ }{\mathrm{h}}^{+}\mathrm{ }\to \mathrm{ }[{\mathrm{L}\mathrm{n}}^{3+}+{\mathrm{h}}^{+}\iff {\mathrm{L}\mathrm{n}}^{4+}]\\ \left[{\mathrm{L}\mathrm{n}}^{3+}+{\mathrm{h}}^{+}\mathrm{ }\iff {\mathrm{L}\mathrm{n}}^{4+}\right]+{\mathrm{e}}^{-}{\to \mathrm{L}\mathrm{n}}^{3+\mathrm{*}}\end{array}$
(3)
Electron trapping at the dopant site followed by hole trapping from a non-dopant site and creation of the excited Ln³⁺

$\begin{matrix} {L n}^{3 +} + e^{-} \to [{L n}^{3 +} + e^{-} {⟺ L n}^{2 +}] \\ [{L n}^{3 +} + e^{-} {⟺ L n}^{2 +}] + h^{+} {\to L n}^{3 + *} \end{matrix}$ $\begin{array}{c}{\mathrm{L}\mathrm{n}}^{3+}+{\mathrm{e}}^{-}\to \mathrm{ }\left[{\mathrm{L}\mathrm{n}}^{3+}+{\mathrm{e}}^{-}{\iff \mathrm{L}\mathrm{n}}^{2+}\right]\\ \left[{\mathrm{L}\mathrm{n}}^{3+}+{\mathrm{e}}^{-}{\iff \mathrm{L}\mathrm{n}}^{2+}\right]+{\mathrm{h}}^{+}{\to \mathrm{L}\mathrm{n}}^{3+\mathrm{*}}\end{array}$
(4)
Electron-hole pair recombination at the dopant site and creation of the excited Ln³⁺

${L n}^{3 +} + [h^{+} + e^{-}] {\to L n}^{3 + *}$ ${\mathrm{L}\mathrm{n}}^{3+}+[{\mathrm{h}}^{+}+{\mathrm{e}}^{-}]{\to \mathrm{L}\mathrm{n}}^{3+\mathrm{*}}$
(5)
Luminescence from the Ln³⁺ excited energy level

${L n}^{3 + *} \to {L n}^{3 +} + h ν$ ${\mathrm{L}\mathrm{n}}^{3+\mathrm{*}}\mathrm{ }\to \mathrm{ }{\mathrm{L}\mathrm{n}}^{3+}+\mathrm{h}\mathrm{\nu }$

It is proposed that the sensitization is most effective with process (4) is in operation. Such is the case in the Zn(Tb)S NPs. In short, the experiments with ZnS/Ln [Ln = Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb], CdS/Ln [Ln = Eu, Tb], and CdSe/Ln [Ln = Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb] NPs reveal generation of Ln semiconductor NP based luminophores with (a) facile incorporation of Ln in the NPs and (b) realization of host sensitized Ln³⁺ emission in ZnS/Ln [Ln = Eu, Tb, Yb], CdS/Ln [Ln = Eu, Tb], and CdSe/Ln [Ln = Eu, Tb] NPs that can be rationalized by a charge trapping mediated dopant emission sensitization mechanism.

Scheme 2

Plausible charge trapping scenarios in doped semiconductor NPs.

II. Modulation of the Electronic Properties of NPs

A. Zn(Tb)S/M NPs: Core Alteration. Modifying an inorganic NP post-synthetically can access nanomaterials of unique properties. A favorable combination of existing and added cation pair can result in cation exchange [7], feasibility of which can be predicted based on thermodynamic consideration [21, 23]. Dimensional advantage additionally ensures a kinetically facile reaction. That is, these reactions oftentimes happen at or near ambient conditions. On the other hand, even when such an exchange is not happening, post-synthetic modification of the NPs can simply tune the surface state composition of the NPs. Considering a large surface-to-volume ratio surface atoms are of significant contribution in dictating the overall properties of the NPs. Thus, understanding the implications of post-synthetic modification process on the luminescence behavior of the NPs is of interest. We first performed experiments in which the 1-thioglycerol (1-TG) capped Zn(Tb)S NPs are post-synthetically modified by Pb²⁺, Hg²⁺, Cd²⁺, Ca²⁺, Mg²⁺, Na⁺, and K⁺ individually [102]. These experiments suggest that the cation exchange of Zn²⁺ by Pb²⁺/Hg²⁺/Cd²⁺ is feasible. Two test cases, Zn(Tb)S/Pb and Zn(Tb)S/Cd, are considered that involves changing the NP’s interior due to cation exchange. Cation exchange reactions are typically performed with comparable or excess amount of incoming cations. We approached studying these reactions in two relative reactant concentration regimes. First one is the pre-cation exchange regime with sub-stoichiometric reactant concentration range and second one is the cation exchanged counterparts where the NPs are treated with equimolar or excess amount of incoming cations.

(1) Zn(Tb)S/Pb NPs. In 2018, we reported experimental observation from the Zn(Tb)S NPs that is post-synthetically treated with Pb²⁺ to form Zn(Tb)S/Pb NPs [102]. The [Zn(Tb)S]:[Pb²⁺] was varied from 1:10⁻⁵–1:10. The elemental composition of these NPs based on EDS measurements is shown in panel (a) of Figure 5. In the range of [Zn(Tb)S]:[Pb²⁺] = 1:10⁻⁵–1:10⁻², Zn and Tb amounts show near constancy. When this ratio reaches 1:10⁻¹, Zn starts getting exchanged by Pb, and the NPs with 1:1 and 1:10 reactant concentration ratio essentially tracks completely cation exchanged NPs. That is, the NPs with [Zn(Tb)S]:[Pb²⁺] = 1:10⁻⁵–1:10⁻², 1:10⁻¹–1:1, and >1:1 monitors NPs with no cation exchange, partially cation exchanged, and completely cation exchanged NPs. The XRD profiles of the Zn(Tb)S and the NPs with [Zn(Tb)S]:[Pb²⁺] = 1:1 are shown in panel (b) of Figure 5. While the Zn(Tb)S NPs exhibit diffraction pattern that is characteristic of a cubic ZnS phase, the Pb containing NPs develop distinct PbS phases with disappearance of ZnS phases. This further corroborates with the cation exchange process. Gradual addition of Pb²⁺ to the Zn(Tb)S NPs with diameter of 3.0 ± 0.5 nm with [Zn(Tb)S]:[Pb²⁺] = 1:10⁻⁴, 1:10⁻² resulted in the Zn(Tb)S/Pb NPs with diameters of 3.0 ± 0.5 nm, 3.2 ± 0.4 nm, respectively. Further addition of Pb²⁺ with corresponding reactant concentration ratio of 1:1 yields Pb(Tb)S NPs with diameter of 3.9 ± 0.6 nm. These indicate that formation of PbS phase following cation exchange from the Zn(Tb)S NPs increases the particle dimension marginally, most likely reflecting the increase in lattice parameter (5.406 Å and 5.936 Å for cubic ZnS and PbS, respectively).

Figure 5

The elemental composition from EDS measurements of the Zn(Tb)S and the Zn(Tb)S NPs that are post-synthetically treated with Pb²⁺ with varying concentrations are shown in panel (a). Panel (b) shows the XRD profiles of the Zn(Tb)S and the NPs with [Zn(Tb)S]:[Pb²⁺] = 1:1. Panels (c)–(e) show the luminescence excitation and emission spectra of the NPs investigated. The trends in quantum yields of ZnS and Tb³⁺ emissions in the NPs studied are shown in panels (f) and (g), respectively. The luminescence spectra from the experiments when the NPs with [Zn(Tb)S]:[Pb²⁺] = 1:1 is treated with an excess of Zn²⁺ and Tb³⁺ are shown in panel (h). The relative energy level schematic of Tb³⁺ with respect to the valence and conduction bands of the host ZnS and PbS are shown in panel (i). Panel (j) summarizes the Tb³⁺ emission quantum yields in different Zn(Tb)S/M [M = Pb, Hg, Cd, Ca, Mg, Na, K] NPs, with [Zn(Tb)S]:[Mⁿ⁺] = 1:10⁻². (Adapted with permission from Reference 102, Copyright 2018 Royal Society of Chemistry).

The luminescence excitation and emission spectra of the Zn(Tb)S and Zn(Tb)S/Pb NPs are shown in panels (c)–(e) of Figure 5. Zn(Tb)S NPs exhibit characteristic broad blue emission from ZnS center and sharp emissions from Tb³⁺ center. All the post-synthetically modified NPs in the pre-cation exchange range show similar spectral pattern, albeit with varying emission efficiency. The excitation spectra, while monitoring the Tb³⁺ emission at 545 nm, reveal a broad profile that has overlap with that obtained by monitoring the ZnS emission at 400 nm. This suggests presence of a host sensitization. In the cation exchanged NPs, the emission from both the ZnS and Tb³⁺ centers are greatly diminished. The emission quantum yield values of ZnS and Tb³⁺ centers are shown in panels (f) and (g) of Figure 5, respectively. It is remarkable to note that even in presence of a small amount of Pb in the NPs, a rich photophysics evolves. For example, the ZnS broad emission decreases with respect to that in the Zn(Tb)S NPs even in the NPs with [Zn(Tb)S]:[Pb²⁺] = 1:10⁻⁴–1:10⁻². On the other hand, most interestingly, Tb³⁺ emission is additionally sensitized in presence of Pb. This effect is more pronounced in the NPs with [Zn(Tb)S]:[Pb²⁺] = 1:10⁻³–1:10⁻². Efforts on detailed understanding of this Pb induced Tb³⁺ emission brightening in ZnS NPs is currently underway in our laboratory. Further increase in [Pb²⁺] adversely affects the Tb³⁺ emission. Two factors contribute to this. First, a reduced Tb content once [Zn(Tb)S]:[Pb²⁺] reaches ~1:1, and second being the unfavourable energy level alignment (vide infra).

A fascinating direction with cation exchange in inorganic NPs is the ability of reverse cation exchange. Alivisatos and co-workers reported transformation of CdSe → Ag₂Se → CdSe NPs by treating the Ag₂Se with an excess of Cd²⁺ [7]. Although this reaction recovers the CdSe NPs, yet their photophysical properties differ to that of the initial CdSe NPs significantly. For example, the initial NPs only exhibit excitonic emission; however the recovered NPs show an additional contribution from a red shifted emission band. Our efforts to treat the NPs with [Zn(Tb)S]:[Pb²⁺] = 1:1 with an excess of Zn²⁺ and Tb³⁺, as shown in panel (h) of Figure 5, indeed recovers emission properties of Zn(Tb)S. However, key differences between the photophysical properties of these NPs with that of the recovered NPs are evident. These include a red shifted ZnS emission and reduced efficiency of Tb³⁺ emission in the recovered NPs. Clearly the synthesized Zn(Tb)S NPs and post-synthetically recovered Zn(Tb)S NPs differ in the surface state composition which could have implications in their emission characteristics.

The electronic properties need to be evaluated to understand the completely diminished Tb³⁺ emission in the NPs [Zn(Tb)S]:[Pb²⁺] ≥ 1:1. In this reaction domain PbS phases form by exchanging Zn²⁺ and to some extent Tb³⁺ from the Zn(Tb)S NPs by Pb²⁺. PbS has a smaller band gap. The relative energy level positions of Tb³⁺ energy levels in ZnS and PbS are shown in panel (i) of Figure 5. This reveals that the trapping of photogenerated charge carriers is not efficient in PbS, especially the luminescent energy level ⁵D₄ would suffer from auto-ionization even if this energy level is populated by some means. Consequently, Tb³⁺ emission in PbS is highly quenched.

A remarkable outcome from the Pb²⁺ added Zn(Tb)S NPs is the Pb induced Tb³⁺ emission brightening. This is interesting considering the difficulty to make Ln³⁺ luminophores brighter that is associated with very low direct absorption efficiency and quenching of their emission by the vibrational overtones of the immediate ligand and solvent molecules. In order to evaluate the specificity, experiments are performed in which the Zn(Tb)S NPs are post-synthetically treated with various other cations (Mⁿ⁺) with [Zn(Tb)S]:[Mⁿ⁺] = 1:10⁻² [M = Hg, Cd, Ca, Mg, Na, K]. The trend in Tb³⁺ emissions from these experiments is shown in panel (j) of Figure 5. A Pb specificity is indeed observed, at least among the cations considered, as the other cations do not provide any such favourable mechanism for Tb³⁺ emission enhancement in the Zn(Tb)S NPs.

(2) Zn(Tb)S/Cd NPs. Cd²⁺ is able to exchange Zn²⁺ from ZnS based nanomaterials and this reaction progression can access the visible spectral window. [103, 104] We undertook experiments in which the Zn(Tb)S NPs are post-synthetically modified by varying concentrations of Cd²⁺ with [Zn(Tb)S]:[Cd²⁺] = 1:10⁻⁴–1:10 [105]. Similar to the concept discussed for the Pb²⁺ treated Zn(Tb)S NPs discussed above, these experiments also cover the pre-cation exchange and cation exchanged NPs. One of the prime objective of this work is to investigate how the structural evolution of the NPs can tune the emission properties of the dopant Tb³⁺ in the Zn(Tb)S NPs.

The elemental composition of the Zn(Tb)S NPs and the NPs with [Zn(Tb)S]:[Cd²⁺] = 1:10⁻⁴–1:10 are shown in panel (a) of Figure 6. The amounts of Zn, Tb, and S remains almost similar in the range of [Zn(Tb)S]:[Cd²⁺] = 1:10⁻⁴–1:10⁻², with the incorporation of Cd in the NPs being evident. Further increase of [Cd²⁺] gradually displaces Zn²⁺ from the NPs and a partial cation exchange proceeds. Even in presence of an excess amount of Cd²⁺, under the reaction condition the exchange of Zn²⁺ by post-synthetically treated Cd²⁺ remains partial. Others researchers also have discussed such a partial cation exchange for Zn²⁺–Cd²⁺ combination [106]. It is important that throughout the reaction, the amount of Tb in the NPs remains similar. That is, exchange of Tb³⁺ by Cd²⁺ is not happening. Thus, these NPs can directly monitor the impact on Tb³⁺ emission that may arise from the NP’s structural changes. The XRD profiles can track the reaction progression, as shown in panel (b) of Figure 6, indicates formation of characteristic cubic crystal phases for both the Zn(Tb)S and partially exchanged NPs. It is to note that the XRD peak positions in the NPs with [Zn(Tb)S]:[Cd²⁺] = 1:1/1:10 does not match completely with the corresponding profile of the Cd(Tb)S NPs. This further corroborates with a partial cation exchange. The unit cell parameters of the different NPs investigated are plotted as a function of composition of the NPs, and are shown in panel (c) of Figure 6. The Vegard law analysis reveals a linear trend, suggesting formation of homogeneous alloy while the Zn(Tb)S NPs are post-synthetically treated with Cd²⁺. Cd²⁺ addition to the Zn(Tb)S NPs with 3.0 ± 0.6 nm diameter and the NPs with [Zn(Tb)S]:[Cd²⁺] = 1:1 produces particles with 3.1 ± 0.6 nm diameter, suggesting no significant alteration of the NP’s dimension following the post-synthetic reaction.

Figure 6

The elemental composition and XRD profiles of the Zn(Tb)S and the Zn(Tb)S NPs that are post-synthetically modified by varying concentrations of Cd²⁺ are shown in panels (a) and (b), respectively. Panel (b) also includes the XRD profile for the Cd(Tb)S NPs. The trend of lattice parameters as a function of composition of the NPs is shown in panel (c). Panels (d) – (g) summarize the luminescence excitation and emission spectra. The trend in Tb³⁺ emission quantum yield is shown in panel (h). Panels (i) and (j) summarize the trends in shorter lifetime component and average lifetime of the broad emission. Panel (k) shows the relative contributions of shorter-lived surface related and longer-lived core related Tb³⁺ emission components in these NPs. (Adapted with permission from Reference [105], Copyright 2019 American Chemical Society).

The luminescence spectra of the Zn(Tb)S NPs and the post-synthetically modified NPs are shown in panels (d)–(g) of Figure 6. In presence of very low [Cd²⁺] in the NPs with [Zn(Tb)S]:[Cd²⁺] = 1:10⁻⁴–1:10⁻³, emission band maxima blue shifts. This suggests significant reorganization of surface trap states can occur even in presence of small amount of Cd in the NPs. Once the reactant concentration ratio reaches [Zn(Tb)S]:[Cd²⁺] = 1:10⁻², NP’s band gap starts decreasing. The emission band maxima thus red shifts. Emissions from these NPs span the UV–visible spectral window. The excitation spectra also reveal this composition change of the NPs with characteristic spectral shift. Monitoring the Tb³⁺ emission band at 545 nm reveal an excitation spectrum that is broad and devoid of sharp direct excitation bands indicative of host sensitized dopant emission. The Tb³⁺ emission quantum yield trend, as shown in panel (h) of Figure 6, shows a decrease in the NPs with [Zn(Tb)S]:[Cd²⁺] = 1:10⁻⁴–1:10⁻³. This decrease suggests that minor alterations in NP’s surface state composition can affect the Tb³⁺ emission in these NPs. Further decrease of Tb³⁺ emission quantum yield in the NPs with [Zn(Tb)S]:[Cd²⁺] = 1:10⁻²–1:10 reflects the formation of Zn–Cd alloyed NPs (vide infra).

Measurements of emission lifetimes provide information on corresponding rates of reactions. There are two emissive centers in Zn(Tb)S, the broad ZnS and sharp Tb³⁺ emission bands. In order to address how the dynamics proceed, the emission lifetimes of both these bands were collected in the Zn(Tb)S and Zn(Tb)S/Cd NPs. The trends in shorter lifetime component and average lifetime from the shorter-lived ZnS emission lifetime measurements are summarized in panels (i) and (j), respectively. A salient feature from the emission lifetimes of the broad band include [Cd²⁺] dependent depopulation kinetics in the NPs with a lengthening with gradual progression of cation exchange. The trend in contributions from shorter-lived surface related and longer-lived core related lifetime components of Tb³⁺ emission is shown in panel (k) of Figure 6. Tracking Tb³⁺ kinetics directly captures significant reorganization in the NP’s surface states, as evidenced by a decrease in the contribution of the shorter lifetime component.

In order to rationalize the photophysical outcomes in the NPs investigated, the relative energy levels that place the dopant Tb³⁺ energy levels with respect to the valence and conduction bands of the host material are constructed, and these are shown in Figure 7. In the Zn(Tb)S NPs, Tb³⁺ ground and luminescent energy levels are suitably placed above and below the valence and conduction bands of the host ZnS, and hence these energy levels can act as hole and electron traps. Subsequent electron-hole pair recombination at these Tb³⁺ related trap sites populates the Tb³⁺ luminescent energy levels, thus generating the ZnS NP host sensitized dopant Tb³⁺ emission from the Zn(Tb)S NPs. In the NPs with [Zn(Tb)S]:[Cd²⁺] = 1:10⁻⁴–1:10⁻³, the Cd only acts as a surface state modulator and dopant moiety. Presence of Cd in ZnS shift the valence and conduction bands by 0.08 and 0.32 eV below their original positions [107]. This can be correlated with observation of an additional band at 305 nm in the luminescence excitation spectrum while monitoring the 365 nm emission of these NPs. Furthermore, shifting of the conduction band can increase the probability of back electron transfer and thus reduce the Tb³⁺ emission quantum yield. Formation of the alloyed Zn_1-xCd_xS in the NPs with [Zn(Tb)S]:[Cd²⁺] = 1:10⁻²–1:10 while extends the Tb³⁺ excitation to lower energy, they yield Tb³⁺ emission to a lesser extent. In these NPs, the band gaps gradually decrease. Thus, in the alloys with gradual exchange of Zn²⁺ by Cd²⁺, the ΔE₂ value gradually decreases. This results in roles of auto-ionization of excited electrons in the Tb³⁺ energy level more facile, thus decreasing their emission. Finally, in the Cd(Tb)S NPs this effect is even more enhanced and the Tb³⁺ emission is reduced.

Figure 7

The relative energy level that positions the Tb³⁺ energy levels with respect to the valence and conduction bands of the Zn(Tb)S NPs, the NPs with [Zn(Tb)S]:[Cd²⁺] = 1:10⁻⁴–1:10⁻³, the NPs with [Zn(Tb)S]:[Cd²⁺] = 1:10⁻²–1:10, and the Cd(Tb)S NPs are shown in panels (a)–(d), respectively. (Adapted with permission from Reference [105], Copyright 2019 American Chemical Society).

B. Zn(Tb)S NPs: Surface Alteration. The experiments discussed above demonstrate the usefulness of post-synthetic reaction strategy to generate Ln doped inorganic NPs and ways to tune their electronic properties. These systems essentially change the NP’s core structure. Another extremely important variable in the quantum confined regime is the NP’s surface state composition. This surface has two components in the form of surface localized atoms and the surface capping ligands. A suitable capping on the NPs confers stability and guides its dispersion nature. In a series of experiments with 1-TG capped Zn(Ln)S [Ln = Sm, Eu, Tb, Dy ] NPs, we identified that this ligand offers aqueous dispersion of the NPs and Zn(Tb)S NPs as the most efficient system with host sensitized Tb³⁺ emission [86]. Correspondingly, it remains elusive to decipher the effect of 1-TG structural alteration on the luminescence behavior of the NPs. This aspect is addressed by performing experiments in which the pH of the aqueous dispersion of the 1-TG capped Zn(Tb)S NPs was controlled post-synthetically. [108] By virtue of the fact 1-TG has ionizable groups; a change in pH of the medium should modify its chemical identity [109].

Panels (a)–(c) of Figure 8 show the luminescence excitation and emission spectra of the Zn(Tb)S NPs as a function of pH that has been adjusted post-synthetically. All the NPs show characteristic broad blue ZnS emission and sharp Tb³⁺ emission bands. Excitation spectra, while monitoring the Tb³⁺ emission at 545 nm, reveal a broad profile with significant overlap to that obtained while monitoring the ZnS centered emission band at 400 nm, suggesting that operation of host sensitization is present in all these NPs. However, the extent of ZnS and Tb³⁺ emissions is clearly different. These trends are shown in panel (d) of Figure 8. Both of these emissions increase with an increase in pH of the medium, with the effect being more prominent in the Tb³⁺ emission. In this line, it is observed that the proportion of direct excitation over host sensitization for Tb³⁺ emission increases in acidic pH. Furthermore, this pH dependent tuning of emission is found to be reversible, as shown in panel (e) of Figure 8. This suggests an importance of structural alterations of surface capping ligand in guiding the observed trend in photophysical properties and that this is not a mere reflection of NP’s surface photochemistry. The Tb³⁺ emission lifetime decay profiles at different pH of the dispersion are shown in panel (f) of Figure 8. Corresponding contributions from individual lifetime components are shown in panel (g) of Figure 8. There is a lengthening of emission lifetime on going from acidic to basic pH. Closer inspection reveals that this effect is specific for the shorter surface related lifetime component on going from pH 4.97 to 6.27, while the other changes show a general effect. Thus, these results clearly demonstrate that the Tb³⁺ emission properties are guided by the alterations in the surface state composition of the NPs.

Figure 8

The luminescence excitation and emission spectra of the Zn(Tb)S NPs as a function of pH that is modified post-synthetically are shown in panels (a)–(c), respectively. Panel (d) summarizes the relative trend of ZnS and Tb³⁺ emissions in these NPs. The reversible nature of pH dependent effect is shown in panel (e). Panels (f) and (g) show the Tb³⁺ emission lifetime decay profiles and the trends of contributions from shorter-lived surface related and longer-lived core related lifetime components. (Adapted with permission from Reference 108, Copyright 2020 American Chemical Society).

In order to understand the trend in Tb³⁺ emission in these NPs, effects of band alignment, dopant local site symmetry, and capping ligand structural alterations are systematically evaluated. The size of the NPs and the corresponding relative energetic, Tb³⁺ emission spectral shape, zeta potential, and IR absorption spectral characteristic of OH stretching band are shown in panels (a)–(b), (c), (d), and (e)–(f) of Figure 9, respectively. The post-synthetic modification of NPs does not result in significant changes in NP dimension and dramatic alterations in Tb³⁺ emission band shapes; indicating that the changes in the relative band alignment and coordination environment are not the governing factors and subtle modifications in the ligand structure is primarily responsible in dictating the photophysical outcome in these NPs. This change can be visualized by zeta potential measurements and in the infrared absorption spectral features. Additionally, a decrease in Tb content in the NPs is observed in acidic medium, as evaluated by the EDS measurements. These analyses strongly suggest that post-synthetic modification of inorganic NPs also has the ability to control dopant emission by altering surface capping ligand structure, even when the NP’s core related variables are relatively unperturbed.

Figure 9

The size of the Zn(Tb)S NPs as a function of pH that is adjusted post-synthetically is shown in panel (a). Panel (b) shows the relative energy level schematic of the Zn(Tb)S NPs in different pH. A comparison of Tb³⁺ emission spectral shape is shown in panel (c). The zeta potential values, and the trends in spectral characteristics of OH stretching absorption band are shown in panels (d) and (e) - (f), respectively. (Adapted with permission from Reference 108, Copyright 2020 American Chemical Society).

Conclusions

Motivated by the unprecedented opportunity of post-synthetic modification of inorganic NPs and the luminescence properties of Ln³⁺, this work summarizes the key outcomes from experiments in which either the semiconductor NPs are treated post-synthetically with Ln³⁺ from solutions or the synthetically Ln³⁺ incorporated (doped) semiconductor NPs are treated post-synthetically by other cations from solutions. Treating ZnS/CdS/CdSe NPs with Ln³⁺ at room temperature incorporates Ln in the NPs in all cases and in the ZnS/Ln [Ln = Eu, Tb, Yb], CdS/Ln [Ln = Eu, Tb], and CdSe/Ln [Ln = Eu, Tb] host sensitized Ln³⁺ emission is realized. These observations can be well rationalized by a charge trapping mediated dopant emission sensitization mechanism. In this model, the Ln³⁺ ground and luminescent energy levels need to be optimally placed with respect to the valence and conduction bands of the host NP, thus they can act as the charge trap centers for the photogenerated hole and electron pairs. The electron-hole pair recombination at the Ln³⁺ related trap sites populates the Ln³⁺ luminescent energy levels, thus generating the optical antenna effect with NPs sensitized Ln³⁺ emission. While post-synthetically modifying the Zn(Tb)S NPs with Pb²⁺ / Cd²⁺, two relative reactant concentration regimes are investigated. These are [Zn(Tb)S]:[Pb²⁺]/[Cd²⁺] = 1:10⁻⁴–1:10⁻² and 1:10⁻²–1:10, which represent pre-cation and cation exchanged NPs, respectively. Remarkably existence of rich photophysics are observed in both the cases, with a Pb specific Tb³⁺ emission brightening in the NPs with [Zn(Tb)S]:[Pb²⁺] = 1:10⁻³–1:10⁻². This observation finds a global interest in the light of brightening an Ln³⁺ luminophore in general, that is interesting due to inefficient direct excitation and environment induced Ln³⁺ emission quenching. In the cation exchanged reaction conditions, it is observed that under the reaction conditions employed Pb²⁺ and Cd²⁺ can completely and partially exchange Zn²⁺ from the Zn(Tb)S NPs, respectively. In Zn(Tb)S/Pb NPs, a decrease in Tb content is also observed, an effect which is found to be absent in the Zn(Tb)S/Cd NPs. Photophysical properties in these NPs can be well tracked with the change in the NPs band gap and the charge trapping mediated processes. Experiments are also performed with the post-synthetically pH modified 1-TG capped Zn(Tb)S NPs. These measurements reveal a dramatic effect of capping ligand structure in controlling both the ZnS and Tb³⁺ emissions reversibly. Systematic analysis reveals that in this process the effects of band gap engineering and drastic modification of dopant local site symmetry is not important, further emphasizing the control from the capping ligand structure.

In short, the results discussed demonstrate that it is possible to generate Ln³⁺ semiconductor NPs based luminophores by using the post-synthetic modification strategy, and the electronic properties of Ln³⁺ centers in the Ln³⁺ doped semiconductor NPs can also be modified by post-synthetic addition of other cations, by either modulating their core or surface structure. Future directions in this research field include development of co-doped NPs and engineering the surface of the NPs for potential multiplex assays and targeted therapy.

Acknowledgments

Financial supports from the Science and Engineering Research Board (SERB), Department of Science and Technology (DST) (SB/S1/PC-040/2013) and the University Grants Commission (UGC)(F. 20-11 (17)/2013(BSR)), India are acknowledged. Saoni Rudra and Madhumita Bhar acknowledge fellowship support from the University of Calcutta and the Council of Scientific and Industrial Research (CSIR), respectively. We also acknowledge the support from CRNN, University of Calcutta. Prasun Mukherjee also acknowledges Dr. Gouranga H. Debnath, Ms. Saoni Rudra, Ms. Madhumita Bhar, Dr. Chad M. Shade, Dr. Daniel N. Lamont, Ms. Robin F. Sloan who have contributed in this project. Prasun Mukherjee sincerely thanks Prof. Stéphane Petoud and Prof. David H. Waldeck for introducing him to the lanthanide-semiconductor NPs project and for many insightful discussions.

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Cite this article as: Rudra S, Bhar M & Mukherjee P 2023. Post-synthetic modification of semiconductor nanoparticles can generate lanthanide luminophores and modulate the electronic properties of preformed nanoparticles. 4open, 6, 8.

All Tables

Table 1

Summary of relevance of ΔE₁, ΔE₂, and ΔE₃ in CdSe and ZnS NPs.

In the text

Table 2

Energy difference considerations for different Ln related to environment induced emission quenching.

In the text

All Figures

	Scheme 1 Different post-synthetic reaction strategies (RT stands for room temperature).
In the text

Figure 1

Luminescence excitation and emission spectra of the ZnS, ZnS/Tb, and ZnS/Eu NPs are shown in panels (a) and (b), respectively. Panel (c) shows the Eu³⁺ emission trend when ZnS NPs are gradually added to the dispersion. The Ln³⁺ emission lifetime decay profiles in the ZnS/Tb and ZnS/Eu NPs are shown in panel (d), with the corresponding profiles of freely floating Ln³⁺ included. Luminescence excitation and emission spectra of the post-synthetically treated CdS/Tb and CdS/Eu NPs are shown in panels (e) and (f), respectively. The electronic absorption spectrum of the CdS NPs is also included in panel (e) for comparison. Panels (g), (h), and (i) show the trends in Eu³⁺ asymmetry ratio, emission lifetime, and emission lifetime distribution, respectively. (Adapted with permission from Reference [87], Copyright 2013 American Chemical Society).

In the text

	Figure 2 Elemental composition of the CdSe/Ln and ZnS/Ln, as determined from the EDS measurements are shown in panels (a) and (b), respectively. (Adapted with permission from Reference [90], Copyright 2019 Elsevier).
In the text

Figure 3

The luminescence excitation and emission spectra of the CdSe/Ln and ZnS/Ln [Ln = Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb] NPs are shown in panels (a)–(g), respectively. The electronic absorption spectra of the CdSe and ZnS NPs are also included in panels (a) and (b) for comparison. The shorter time decay kinetics of the ZnS and ZnS/Ln [Ln = Tb, Eu, Pr, Ho] to monitor the NP’s intrinsic depopulation rates are shown in panel (h). Panel (i) shows the longer time decay profiles of the CdSe/Ln and ZnS/Ln [Ln = Tb, Eu] NPs to monitor the Tb³⁺ and Eu³⁺ depopulation rates. Panel (j) shows the lifetime distribution analysis of the Tb³⁺ and Eu³⁺ emissions. (Adapted with permission from Reference [90], Copyright 2019 Elsevier).

In the text

Figure 4

Relative energy level schematic with placement of Ln³⁺ ground and luminescent energy levels with respect to the valence and conduction bands of the host material is shown. Ground and luminescent energy levels in each case are represented by black and blue solid lines, respectively. Prominent luminescence transitions are shown with downward arrows in each case. Ln²⁺ ground energy levels are indicated by green horizontal lines. These energy levels are positioned following a method proposed by Dorenbos [96, 97] and later adopted by us, [78–81, 90, 98] and relies on inputs of host independent universal binding energy trend of Ln³⁺ and the charge transfer energy from anion valence band of host material to Ln³⁺ moieties. (Adapted with permission from Reference [90], Copyright 2019 Elsevier).

In the text

	Scheme 2 Plausible charge trapping scenarios in doped semiconductor NPs.
In the text

Figure 5

The elemental composition from EDS measurements of the Zn(Tb)S and the Zn(Tb)S NPs that are post-synthetically treated with Pb²⁺ with varying concentrations are shown in panel (a). Panel (b) shows the XRD profiles of the Zn(Tb)S and the NPs with [Zn(Tb)S]:[Pb²⁺] = 1:1. Panels (c)–(e) show the luminescence excitation and emission spectra of the NPs investigated. The trends in quantum yields of ZnS and Tb³⁺ emissions in the NPs studied are shown in panels (f) and (g), respectively. The luminescence spectra from the experiments when the NPs with [Zn(Tb)S]:[Pb²⁺] = 1:1 is treated with an excess of Zn²⁺ and Tb³⁺ are shown in panel (h). The relative energy level schematic of Tb³⁺ with respect to the valence and conduction bands of the host ZnS and PbS are shown in panel (i). Panel (j) summarizes the Tb³⁺ emission quantum yields in different Zn(Tb)S/M [M = Pb, Hg, Cd, Ca, Mg, Na, K] NPs, with [Zn(Tb)S]:[Mⁿ⁺] = 1:10⁻². (Adapted with permission from Reference 102, Copyright 2018 Royal Society of Chemistry).

In the text

Figure 6

The elemental composition and XRD profiles of the Zn(Tb)S and the Zn(Tb)S NPs that are post-synthetically modified by varying concentrations of Cd²⁺ are shown in panels (a) and (b), respectively. Panel (b) also includes the XRD profile for the Cd(Tb)S NPs. The trend of lattice parameters as a function of composition of the NPs is shown in panel (c). Panels (d) – (g) summarize the luminescence excitation and emission spectra. The trend in Tb³⁺ emission quantum yield is shown in panel (h). Panels (i) and (j) summarize the trends in shorter lifetime component and average lifetime of the broad emission. Panel (k) shows the relative contributions of shorter-lived surface related and longer-lived core related Tb³⁺ emission components in these NPs. (Adapted with permission from Reference [105], Copyright 2019 American Chemical Society).

In the text

Figure 7

The relative energy level that positions the Tb³⁺ energy levels with respect to the valence and conduction bands of the Zn(Tb)S NPs, the NPs with [Zn(Tb)S]:[Cd²⁺] = 1:10⁻⁴–1:10⁻³, the NPs with [Zn(Tb)S]:[Cd²⁺] = 1:10⁻²–1:10, and the Cd(Tb)S NPs are shown in panels (a)–(d), respectively. (Adapted with permission from Reference [105], Copyright 2019 American Chemical Society).

In the text

Figure 8

The luminescence excitation and emission spectra of the Zn(Tb)S NPs as a function of pH that is modified post-synthetically are shown in panels (a)–(c), respectively. Panel (d) summarizes the relative trend of ZnS and Tb³⁺ emissions in these NPs. The reversible nature of pH dependent effect is shown in panel (e). Panels (f) and (g) show the Tb³⁺ emission lifetime decay profiles and the trends of contributions from shorter-lived surface related and longer-lived core related lifetime components. (Adapted with permission from Reference 108, Copyright 2020 American Chemical Society).

In the text

Figure 9

The size of the Zn(Tb)S NPs as a function of pH that is adjusted post-synthetically is shown in panel (a). Panel (b) shows the relative energy level schematic of the Zn(Tb)S NPs in different pH. A comparison of Tb³⁺ emission spectral shape is shown in panel (c). The zeta potential values, and the trends in spectral characteristics of OH stretching absorption band are shown in panels (d) and (e) - (f), respectively. (Adapted with permission from Reference 108, Copyright 2020 American Chemical Society).

In the text

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Post-synthetic modification of semiconductor nanoparticles can generate lanthanide luminophores and modulate the electronic properties of preformed nanoparticles

Introduction

Results and discussion

I. CA/Ln NPs: generation of Ln3+ doped semiconductor NP luminophores

II. Modulation of the Electronic Properties of NPs

Conclusions

Acknowledgments

References

All Tables

All Figures

I. CA/Ln NPs: generation of Ln³⁺ doped semiconductor NP luminophores